1. Field of the Invention
The present invention relates to a fastening element having a core of a relatively hard carburized steel and a ferritic rim zone of a less hard, low-carbon steel and including a stem, a head provided at one end of the stem, and a tip provided at an opposite end of the stem.
2. Description of the Prior Art
Fastening elements of the type described above, such as nails, bolts and the like and which are made of steel, are used for attaching or fastening objects to hard materials such as concrete, metal, stone and the like. The fastening elements have a stem provided at its end, which faces in the setting direction, with a tip, and at its opposite end with a head having a diameter greater than that of the stem. The driving-in process is effected by applying blows to the head or by driving the fastening element in with a combustion energy-actuated setting tool.
German Publication DE 4139 653A1 discloses a fastening element, a nail, having a head and a stem provided at its end remote from the head with a tip. Whereas the head is formed of a deformable steel, the stem is formed of steel having a high mechanical rigidity and is deformable only over a limited circumference.
U.S. Pat. No. 5,851,153 discloses a steel pin having a ferritic outer zone extending coaxially with respect to an inner martensitic zone.
A drawback of fastening elements of the types described above and which have a relatively soft ferritic outer zone consists in a possibility that the tip would bend-up, become kinked upon striking an added material (e.g., pebbles) in concrete. As a result, a holding force of a fastening element is reduced.
Accordingly, an object of the present invention is to provide a fastening element in which the drawbacks of conventional fastening elements are eliminated.
Another object of the present invention is to provide a fastening element having a high holding force even in a non-uniformly hard material such as, e.g., concrete.